Combined air intake and combustion gas vent terminals, sometimes referred to as vent/intake terminals, have long been used with fuel fired heating appliances, particularly with side wall vented gas fireplaces and furnaces. Combined vent/intake terminals typically comprise concentrically mounted vent and intake conduits, with a large intake conduit disposed around a smaller vent conduit. The terminal is installed in an exterior wall of a building, with the intake and vent opening exterior to the building. The recent popularity of the side wall vented furnaces, also known as horizontally vented or direct vented furnaces is due to the ease with which the required air intake and the flue systems may be installed in the building. Correspondingly, there has been an increased demand for vent/intake terminals because such devices simplify installation effort and cost, as only a single fixture need be installed.
One problem commonly encountered with vent/intake terminals is an unwanted recirculation of combustion gases into the terminal intake which reduces the efficiency of combustion in the fuel fired appliance. Such recirculation is principally caused by the close proximity of the intake and vent openings, and one approach to solving this is the use of an anti-mixing baffle to isolate the two.
Another problem that can occur with such systems is that of wind induced pressure effects on the operation of the terminal. That is, the different orientations of the intake and vent openings in the prior art terminals may result in wind induced pressure differentials between the openings. This is undesirable because it modifies the pressure differential generated by the appliance between the intake and vent openings. The combustion pressure differential, which causes intake air to be induced into the appliance and combustion gas to be expelled therefrom, is carefully balanced in high efficiency furnaces to permit an efficient combustion of fuel in the appliance.
To complicate matters, the preferred combustion pressure differential varies with the type of system involved. For example, with oil fired furnaces, referred to as positive pressure furnaces, it is relatively small as compared with that of a draft induced furnace, wherein the speed and pressure at which the draft inducer fan operates reduces the sensitivity of the vent/intake system to wind induced pressure and balance across the intake and vent outlet.
Another problem which can occur with concentric vent/intake terminals is that of over-cooling of the exhaust gas. That is, during conditions of prolonged extreme cold weather, the air intake pipe can cool the exhaust gas to an extent that frost can build up on the inner wall of the exhaust pipe near the outlet, eventually shutting down the furnace.
Another problem that the applicants have encountered with the prior art vent/intake terminals is that they may be of too great a length for a particular installation. That is, when the vent pipes run perpendicular to the floor joists, which are typically spaced 16 inches apart, then the vent/intake terminal cannot be installed if it is substantially greater than 16 inches in length.
What is needed is a vent/intake terminal that overcomes these problems and is easy to install and effective in use.